The present invention relates to a recirculating charge transfer magnetic field sensor.
Conventional semiconductor magnetic field sensors typically operate as a Hall cell. In the operation of Hall-effect components, an electrical potential difference is known to result from charge-carrier drift in a magnetic field. As shown in FIG. 1, the Hall cell 10 includes a resistive sheet 12 or substrate, an input conductive contact 14, an output conductive contact 16, and sense contacts 18. The cell 10 operates to measure the displacement of the carriers in the resistive sheet in response to an applied magnetic field, Conventional Hall cells are microscopic plates, for example, the thickness being t=10 .mu.m, the length l=200 .mu.m and the width w=100 .mu.m. A bias voltage V is applied to the plate via two current contacts C1 and C2, The bias voltage creates an electric field E.sub.e and forces a current I. If the plate is exposed to a perpendicular magnetic induction B, the Hall electric field E.sub.H occurs in the plate. The Hall electric field gives rise to the appearance of the Hall voltage V.sub.H between the two sense contacts S1 and S2.
Charge transport of this type depends on the properties of the semiconductor used and on the environmental conditions. Any change in the semiconductor properties or the environment causes a change in the output signal of the Hall-effect component. Hall-effect components are particularly sensitive to surface effects, encapsulation stress, light, temperature, changes in the material doping density gradient, etc. Therefore, to obtain high-precision magnetic field sensors, the Hall-effect component must be made of a special semiconductor material, including a special encapsulation, and by means of non-standard process steps. Furthermore, external temperature variations must be compensated for. For this reason, it is difficult to integrate a good Hall-effect component into a signal processing circuit.
In addition, errors in measurement occur due to the inability to create precise positioning of charge contacts. Positioning errors of the contacts in the order of micrometers give rise to millivolt output offsets. Also, the ratio of sensitivity to accuracy in such devices remains constant with device geometric changes.
More recently, charge-coupled device (CCD) magnetic field sensors have been developed to improve the precision for measuring magnetic fields. Examples of such devices are described in U.S. Pat. No. 3,906,359 and U.S. Pat. No. 5,194,750, both incorporated herein by reference. In these prior art CCD magnetic field sensors, the measurement arises from a lateral redistribution of carriers within a moving charge packet. The redistribution is caused by the well known Lorentz force acting upon the carriers, which is proportional to their velocity, charge, and the magnitude of the magnetic field. In the devices described in the '750 patent, the potential difference between the ends of the packet (which arises from that redistribution of carriers) is sensed by means of a pair of contacts, centrally located with respect to a series of gates which propagate the packet. In the devices described in the '359 patent, the measure of the magnetic field is given by the difference in the number of carriers arriving at two contacts situated at the end of the array of propagation gates and at the ends of the charge packet, i.e. by the difference in the two currents observed to exit the two contacts.
Unfortunately, the prior art devices lack high measuring sensitivity and accuracy due to geometric limitations and the physics of semiconductor materials. The ultimate sensitivity of such devices is proportional to the width of the propagating charge packet and its velocity. Increasing the width of the packet in turn increases (as the square of the increase in width) the time required for the redistribution to fully occur. Accordingly, for a device of a given length, increasing the velocity (decreasing the time for the packet to traverse the device) or increasing its width will decrease the percentage of full redistribution that occurs yielding no net increase in sensitivity. The redistribution time constant, in fact, involves the carrier mobility which is strongly a function of temperature. The sensitivity that is achieved for such devices, will thus be also affected by temperature and for realistically sized devices, fall far short of the expected sensitivity. In these prior art devices, the carrier mobility temperature coefficient will also affect proportionately, the current observed to exit the contacts and hence their difference. Since that difference is the measurement output, it will have a commensurate additional error.
It is therefore an object of the present invention to provide a CCD magnetic field sensor which maximizes sensitivity and precision, while maintaining realistic limits on device dimensions.
It is a further object of the present invention to provide a CCD magnetic field sensor which operates to ensure that the output potential settles to a value indicated by the Faraday equation by accommodating full lateral redistribution of carriers within propagating charge packets.
It is another object of the present invention to provide a CCD magnetic field sensor which utilizes a recirculation technique to accommodate full lateral redistribution of carriers in the charge packets.